JPS6213349Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an induction heating cooker, and particularly to a small object detection device therefor.

Traditionally, in induction heating cookers that use high-frequency inverters to send high-frequency current through a heating coil and generate a high-frequency magnetic field to heat a metal pot, small object detection has been used to prevent knives and kitchen knives from being accidentally heated. equipment was provided. Conventional small object detection devices utilize the fact that the inverter circuit input current is approximately proportional to the surface area of the load metal, and if the input current is small, it is determined that the load is a small object. However, when controlling the output by changing the conduction/nonconduction ratio of the switching semiconductor of the inverter circuit, that is, the duty ratio, the inverter circuit input current decreases when the induction heating output is lowered, and there is a drawback that it cannot be determined that the load is a small load. Ta. The present invention improves the conventional drawbacks and realizes an induction heating cooker equipped with a small object detection device that can control output by continuously changing the conduction duty of a switching semiconductor. A detailed explanation will be given below according to the drawings.

FIG. 1 shows an embodiment of an inverter device for an induction heating cooker according to the present invention, and FIG. 2 is a block diagram showing an embodiment of a control circuit according to the present invention. Fig. 3 shows waveforms of various parts showing the control method of the inverter circuit according to the invention, Fig. 4 is a characteristic diagram showing the small object detection method according to the invention, and Fig. 5 shows a specific example of the small object detection circuit according to the invention. An example is shown.

In FIG. 1, an AC voltage is applied from a low frequency AC power source 1 to a rectifier circuit 2 and converted into a DC voltage to constitute a DC power source. An inverter circuit 3 is connected between the output terminals of the rectifier circuit 2 to convert DC power into high frequency power. The inverter circuit 3 has an input capacitor 30 connected between DC input terminals, and a series circuit of a heating coil 31 and a switching semiconductor 32 connected in parallel with the input capacitor 30. A damper diode 33 is connected in antiparallel relationship with the switching semiconductor 32, and a resonance capacitor 34 is connected in parallel with the heating coil 31. Resonance capacitor 3
4 operates in the same way even if it is connected in parallel with the switching semiconductor 32. The inverter circuit 3 is controlled by a control circuit 4, and includes input current detection terminals 40a and 40b connected to an input current transformer 40 that detects the input current of the inverter circuit 3.
an input DC voltage detection terminal 41a that detects the input DC voltage of the switching semiconductor 32; a collector voltage detection terminal that detects the applied voltage from the DC terminal of the switching semiconductor 32;
A common terminal 41c connected to the DC-terminal, output terminals 42a and 42b connected to the control terminal of the switching semiconductor 32, and an output control volume 4.
Input setting terminals 43a and 43b connected to the input terminal 3 are provided. FIG. 2 shows a block diagram showing one embodiment of the control circuit according to the present invention. Input DC voltage detection terminal 41a and collector voltage detection terminal 4
Comparing the voltages from 1b, the collector voltage V _CE is
A voltage detection circuit 44 generates a trigger signal V _t when the input DC voltage becomes lower than the input DC voltage V _DC , and a pulse width control circuit (PWM circuit for short) 45 generates a variable pulse synchronized with the output signal V _t . Performs basic oscillation control. FIG. 3 shows waveforms of various parts showing the basic oscillation control method according to the present invention.
The explanation will be given below with reference to FIG. Third
In Figure A, the solid line indicates the collector voltage _VCE of the switching semiconductor 32, and the two-dot chain line indicates the input DC voltage VCE.
_DC . B is the waveform compared with V _DC , and C
is the time t _O when V _CE becomes smaller than V _DC ,
This figure shows how the synchronizing pulse V _t is generated. D and E show the waveforms of the pulse width control circuit 45, and by comparing the sawtooth wave V _r synchronized with the synchronizing pulse V _t and the pulse width setting signal V _S , a pulse width control signal V _P such as E is obtained. to occur. By controlling the output pulse width T _O of the pulse width control signal V _P , the conduction pulse width of the switching semiconductor 32 is controlled, and the heating output of the inverter circuit 3 is controlled. The output signal V _P of the PWM circuit 45 is applied to the gate circuit 46, and the output signal of the gate circuit 46 is applied to the drive circuit 47.
The drive circuit 47 amplifies the signal to sufficiently turn on and off the switching semiconductor 32, and sends the signal to the switching semiconductor 32 at its output terminals 42a, 4.
2b is connected. When the switching semiconductor 32 is a power transistor, base currents I _B1 and I _B2 such as F are driven. G is the forward current I _C of the switching semiconductor 32, and H is the waveform of the current I _L of the heating coil 31. The conduction pulse width T ₁ of the switching semiconductor 32 is almost equal to the pulse width T _O of the pulse width control signal V _P , and the off-state pulse width T ₂ is a resonant waveform of the heating coil 31 and the resonance capacitor 34 and is almost constant. It is. Therefore, by changing _T1 , the effective current of the switching semiconductor 32 and the heating coil 32 decreases, and the output can be controlled. The control circuit 4 according to the present invention controls the input current to be constant, and applies the input current signal from the current transformer 40 to the input current detection circuit 48 to convert it into a voltage signal corresponding to the input current. The output signal of the input current detection circuit 48 is applied to a comparison amplifier circuit 49 and compared with the output signal of the input setting circuit 50.
A pulse width setting signal V _S is applied to the PWM circuit via the pulse width limiting circuit 51 . A collector voltage _VCE signal from the collector voltage detection terminal 41b,
The output signal of the input detection circuit 48 is transmitted to the small object detection circuit 52.
In addition, a comparison is made with the small load, and the oscillation start/stop circuit 53 is operated based on the output signal.
The gate circuit 46 is controlled to stop the conduction of the switching semiconductor if it is a small load. The output signal of the oscillation start/stop circuit 53 is also applied to a start pulse width control circuit 54, which controls the oscillation start pulse width limiting circuit 54 to start the switching semiconductor from a narrow pulse width.

Figure 4 shows the relationship between the input current I _io and the collector voltage V _CE when the input current of the inverter circuit is controlled by changing the conduction pulse width of the switching semiconductor. In the suitability pot, the solid line L is the accessory load. In other words, with a small load, the collector voltage V _CE will necessarily be higher even if the input current is the same. Therefore, the boundary line of K shown by the two-dot chain line becomes the small object detection level for determining the small object and the appropriate load. The small object detection level shown at K is proportional to the input current _Iio , to which a bias voltage _VB is applied.

FIG. 5 shows a specific embodiment of the small object detection circuit according to the present invention, in which the collector voltage detection terminal 41b
A signal proportional to the collector voltage V _CE is detected by the forward voltage detection circuit 52A having the input terminal VCE, and is applied to one input terminal of the small object detection circuit 52B. The forward voltage detection circuit 52A includes a dividing resistor 520a,
A voltage signal is applied to the integrating circuit of the smoothing capacitor 522 and the discharge resistor 523 via the diode 521 from the connection point between the smoothing capacitor 522 and the discharge resistor 523. The voltage of the integrating circuit is applied to the + input terminal of the voltage comparator 525 via the input resistor 524, and from the output terminal of the voltage comparator 525 to the + input terminal of the voltage comparator 525 via the diode 526 and feedback resistor 527. connected to provide positive feedback.

The input current detection circuit 48 converts the voltage generated across the resistor 480 and the resistor 480 connected between the output terminals of the current transformer 40 into direct current using a rectifier circuit 481, and applies the DC current through a smoothing capacitor 482 and a resistor 483 to an integrating circuit. A series circuit of a resistor 484a and a resistor 484b is connected between the DC low voltage V _CC of the control circuit 4 and the ground, and a capacitor 485 is connected in parallel to the resistor 484b to form a DC bias power supply. Connect the bias power supply between earth. The positive voltage between the ground of the smoothing capacitor 482 changes approximately in proportion to the current of the current transformer 40, and becomes only the bias voltage when the input current _Iio is zero. The output signal of the input current detection circuit 48 is sent to the voltage comparator 525 and the comparison amplifier circuit 4.
Added to 9. A voltage comparator 525 compares the output signal of the input current detection circuit 48 and the output signal of the forward voltage detection circuit 52A, and compares the output signal with the output signal of the input current detection circuit 48.
When the output signal of 2A is large, the voltage comparator 52
The output signal of the inverter circuit 3 becomes H level, and due to positive feedback, the output signal of the small object detection circuit 52 maintains the H level, operates the oscillation start/stop circuit 53, stops the oscillation of the inverter circuit 3, and detects the small object load. stop heating.

In the load characteristics shown in Fig. 4, the input current _Iio
The reason why the collector voltage V _CE does not become zero even when V CE is set to zero is that the collector voltage V _CE is dependent on the input DC voltage V
This is because _DC is added. If the voltage of the heating coil 31 of the inverter circuit 3 is V _L , then V _DC =V _C +V
_CE , and V _CE =V _DC -V _L. Input current I _i
When _o is zero, V _L =0. Therefore, V _DC =V _CE .

In FIG. 4, if the forward voltage detection circuit is configured such that the bias voltage V _B is equivalently subtracted from the collector voltage V _CE , the small object detection level and the input current I _io
The relationship is linear, and the input current detection circuit 48
Therefore, there is no need to apply a DC bias. That is, by applying a DC bias to the input current detection circuit 48, or by applying a DC bias to the input current detection circuit 52A,
There are two ways to reduce the DC bias. In the forward voltage detection circuit 52A, the diode 5
A Chener diode may be connected in series with 21.

As described above, the present invention detects small objects by comparing the forward voltage of the switching semiconductor and the input current of the inverter circuit.In particular, in order to maintain the small object detection level in an appropriate relationship, the input current detection circuit, Alternatively, a bias is applied to a forward voltage detection circuit of a switching semiconductor.

According to the present invention, when controlling the induction heating output by controlling the conduction pulse width or conduction duty of the switching semiconductor, even if the small object detection level is set in an appropriate relationship and the input voltage is limited to about 100 W, the small object load This makes it possible to compare the appropriate load. Therefore, with the conventional method, small objects could not be detected when the output was performed continuously, but the present invention enables continuous output control.

Note that the inverter circuit is not limited to the embodiment of the present invention, and any inverter circuit using almost any switching semiconductor may be used.

In addition, the voltage detection method for switching semiconductors is
It can be divided into resistors and has the advantage of being inexpensive.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment of an inverter device for an induction heating cooker according to the present invention, Fig. 2 is a block diagram showing an embodiment of a control circuit according to the present invention, and Fig. 3 is a circuit diagram of an inverter circuit according to the present invention. FIG. 4 is a characteristic diagram showing the small object detection method according to the present invention, and FIG. 5 is a specific circuit diagram of the small object detection circuit according to the present invention. 1... Low frequency AC power supply, 2... Rectifier circuit, 3...
...Inverter circuit, 30...Input capacitor, 3
1...Heating coil, 32...Switching semiconductor, 34...Resonance capacitor, 47...Drive circuit.

Claims (1)

[Claims for Utility Model Registration] (1) Consisting of an inverter circuit that converts DC power into high-frequency power and its control circuit, the inverter circuit includes a heating coil, a switching semiconductor,
and a resonant inductor, the control circuit comprising:
an input current detection circuit that detects an input current of the inverter circuit; a forward voltage detection circuit of the switching semiconductor; and an accessory detection circuit that compares an output signal of the input current detection circuit with an output signal of the forward voltage detection circuit. An induction heating cooker comprising: a bias voltage applied to the input current detection circuit or the forward voltage detection circuit. (2) The induction heating cooker according to claim 1, wherein the input current detection circuit includes an integrating circuit including a smoothing capacitor and a resistor, and a DC bias voltage is applied to the integrating circuit. .